The present invention relates to a continuous process, a plant and a reactor for the preparation of high-purity silicon tetrachloride or high-purity germanium tetrachloride by treatment of the silicon tetrachloride or germanium tetrachloride to be purified, which is contaminated with at least one hydrogen-containing compound, by means of a cold plasma and subsequent fractional distillation of the treated phase.
Silicon tetrachloride (SiCl4) and germanium tetrachloride (GeCl4) are used, inter alia, for producing optical fibres. For these applications, SiCl4 having a very high purity is required. Efforts are likewise made to prepare germanium tetrachloride in very pure, in particular high-purity, form.
Here, hydrogen-containing compounds are a considerable disadvantage, even if they are present in only ppm amounts. However, silicon tetrachloride frequently contains hydrogen-containing trace components or secondary components, e.g. HCl, —Si—OH-containing species, —C—H-containing species and Si—H-containing species. An analogous situation applies to GeCl4.
In the case of hydrogen-containing impurities in silicon tetrachloride, a distinction has to be made between impurities which are difficult to separate off and those which are easy to separate off. HCl, for example, can be separated from silicon tetrachloride down to the region of <1 ppm by weight by simple fractional distillation. On the other hand, hydrocarbons in particular but also chlorinated hydrocarbons and possibly corresponding compounds such as silanes bearing alkyl groups cannot be separated off down to the region of <1 ppm by weight by simple fractional distillation.
Possible ways of removing hydrocarbons, chlorinated hydrocarbons and corresponding compounds such as silanes bearing alkyl groups from silicon tetrachloride have been known for a long time.
Thus, silicon tetrachloride containing the abovementioned components can, according to U.S. Pat. No. 4,372,834 and EP 0 488 765 A1, be treated in the presence of chlorine with UV radiation in the wavelength range from 200 to 380 nm and the chlorination products obtained can subsequently be separated from SiCl4 by fine distillation. A substantial disadvantage of this process is that the plant components come into contact with chlorine gas, which is added in considerable amounts according to EP 0 488 765 A1, and are thus subjected to particularly severe corrosion, which inevitably leads to frequent shutdowns of the plant. In addition, the chlorine to be added likewise has to meet very high purity requirements. Both result in high operating costs for the plant. A further particular disadvantage is the particularly poor energy efficiency of UV radiation sources proposed, for example, by EP 0 488 765 A1. This results in particularly long treatment times, which likewise leads to high costs.
A general process for purifying halogen compounds and hydrogen compounds of silicon is likewise known (DE-B 10 58 482). Thus, chlorosilanes and bromosilanes can be treated by addition of a reducing agent such as hydrogen, silicon, sodium, aluminum or zinc under the action of a gas discharge, in particular a dark gas discharge, forming, as a result of free radical formation and combination of free radicals, relatively high molecular weight compounds in which the elements carbon, boron or phosphorus can be incorporated in relatively high molecular weight compounds of chlorosilicon and which are separated off by distillation. A particular disadvantage of this process is the fact that a reducing agent has to be added. In particular, DE-B 10 58 482 teaches the addition of hydrogen as reducing agent in the purification of an SiCl4 fraction.
Plasma technology has a special place in the generation of ozone from oxygen or air in an ozonizer (EP 0560 166 A1, WO 89/12021, WO 97/09268, WO 01/07360, WO 02/096798, WO 04/046028).
The earlier German patent application 10 2004 037 675.1 teaches a continuous process for the preparation of high-purity silicon tetrachloride or high-purity germanium tetrachloride by treatment of silicon tetrachloride or germanium tetrachloride contaminated with at least one hydrogen-containing compound by means of a cold plasma and subsequent fractional distillation of the phase which has been treated in this way. A cold plasma can in principle also be generated using ozonizer systems. However, reactor systems for producing high-purity silicon tetrachloride or germanium tetrachloride generally require the use of spacers for fixing a precise distance between the electrodes and the dielectric. Furthermore, suitable spacers are only moderately resistant to SiCl4 or GeCl4.